Set sourceCompatibility to 1.6

[mojavelinux]

I just checked and this plugin seems to work with the sourceCompatibility set to 1.6 (all tests pass). You and I both want to migration to JDK8, but the fact is there are many builds on the world that still need to be run on JDK6 and setting the sourceCompatibility to 1.7 means we can test those builds when this plugin is anywhere in the Gradle build. That's a problem. I think we need to support 1.6 until we can rid it from the world.

[rtyler]

@mojavelinux now's your time to shine, cook up a pull request! :D

[mojavelinux]

\o/

Thanks Schalk!